1. Field of the Invention
The present invention relates to a fabrication method of a semiconductor device and, particularly, to a method of selectively etching non-doped polysilicon in a semiconductor device having doped polysilicon and non-doped polysilicon.
2. Description of Related Art
In the fabrication of a semiconductor device such as bipolar transistor, the etching step has been important and indispensable in forming a desired pattern.
For example, in case that a semiconductor device includes a non-doped polysilicon film and a doped polysilicon film, for example, boron-doped polysilicon, there is situation to need to remove selectively the non-doped polysilicon film. Such a situation is performed a wet etching by using a etching liquid.
In the wet-etching step of polysilicon mentioned above, in order to perform an anisotropic etching utilizing a selection ratio between the boron-doped polysilicon film and the non-doped polysilicon film, the etching selection ratio must be high. In order to obtain such high etching selection ratio, it has been usual to use hydrazine which has high alkalinity (pH=12.about.14) and has high reducing capability. However, since the use of hydrazine was inhibited due to its adverse effect to the human body and the ecological system (cause of variability and malformation) and, nowadays, TMAH (tetramethylammonium hydroxide) is used as an etching liquid instead of hydrazine.
Hoever, TMAH can not etch non-doped polysilicon with selectivity as high as that of hydrazine. Further, since the use of TMAH may result in an increase of TOC (Total Organic Concentration), a considerable cost is required for the waste liquid processing due to the recent restriction of the reference value of drain water.